As an ink jet head that performs recording using an ink jet recording system (liquid ejecting recording system), the head having a following structure is typical. Specifically, it is an ink jet head having a discharge port for discharging the ink, an ink flow path communicated with the discharge port and including a pressure generating element for discharging the ink, a substrate on which the pressure generating element is formed, and an ink flow path wall for forming the ink flow path by joining with the substrate.
As a process for manufacturing such a ink jet head, there has been known a process, for example, for forming an ink flow path comprising forming a trench for forming a fine ink flow path on a plate of glass, a metal or the like using processing means, such as cutting and etching, and then, joining to the plate on which the trench is formed a substrate for an ink jet head furnished with a pressure generating element for ejecting the ink. However, in such a conventional process for manufacturing an ink jet head, when the trench is formed using a cutting process, it is difficult to flatten the internal wall surface of the trench, and since the chip or crack of the plate is easily produced, the product yield is not so high. On the other hand, when the trench is formed by etching, there are disadvantages in that it is difficult to make the etching conditions uniform for all the trenches for forming ink flow path, and the process is complicated causing the rise of manufacturing costs. Therefore, in any of these processing means, it is difficult to routinely fabricate ink jet heads having a uniform ink flow path shape, and the obtained ink jet heads tend to have variation in printing characteristics. Furthermore, when the plate on which the above-described trench for forming an ink flow path is joined to the substrate for the ink jet head on which a pressure generating element for ejecting the ink is provided, it is difficult to align the trench with the pressure generating element. Therefore, the above-described conventional process for manufacturing an ink jet head is not suitable for the mass-production of high-quality ink jet heads.
In order to solve such problems in the prior art, in U.S. Pat. No. 4,450,455, there is proposed a process of forming a discharge port comprising providing a dry film consisting of a photosensitive resin material of a substrate for an ink jet head on which the pressure generating element is formed, forming a trench for forming an ink flow path on the dry film using a photolithographic process, joining a top board, such as a glass sheet to the substrate for the ink jet head on which the trench has been formed using an adhesive or the like, and mechanically cutting the end surfaces of the obtained joined body.
According to this process, since the trench for forming the ink flow path is formed using a photolithographic process, it can be accurately formed, and in addition, since the trench has been formed on the substrate for the ink jet head on which the pressure generating element has been provided, an accurate alignment is not necessarily required, and the substrate for the ink jet head can be easily joined to the top board.
However, even using this process, there are problems wherein (1) the above-described adhesive may drop into the ink flow path when the top board is joined to the substrate for the ink jet head, and the shape of the obtained flow path may be deformed; (2) chips may enter into the ink flow path when the above-described joined body is cut for forming the discharge port, and in this case, the obtained ink jet head may be clogged; and (3) since the portion of the joined body to become the ink flow path is hollowed, a part of the discharge port formed by cutting may be cracked when the joined body is mechanically cut.
As processes of solving such problems, U.S. Pat. Nos. 4,657,631, 5,331,344 and 5,458,254 disclose a process wherein a soluble resin layer is provided in the portion to become the ink flow path, a coating resin layer for forming an ink flow path wall to coat the resin layer of the soluble resin layer in the state wherein the soluble resin layer has been provided, and the above-described soluble resin layer is removed. Thereby, the adhesive does not drop into the portion of the ink flow path, and the shape of the ink flow path can be accurately formed. Further, since the portion to become the ink flow path is filled with the soluble resin when the above-described substrate for the ink jet head on which the ink flow path pattern is provided is cut, the possibility of the entering of chips in the ink flow path, and the occurrence of cracks in a part of the discharge port formed by cutting can be reduced. As the above-described soluble resin, a positive type resist is used in view of the ease of removal. The positive type resist forms patterns utilizing difference in dissolving rates between the exposed portion and the unexposed portion, and in any manufacturing processes, the portion of the ink flow path is dissolved and removed after exposure.
It is described that in these processes, the formation of the coating resin layer for forming the ink flow path wall provided on the ink flow path pattern is performed using so-called solvent coating. Solvent coating is a method of applying a predetermined resin for coating after dissolving in a solvent, and spin coating is a typical example. Spin coating has an advantage that the film thickness can be easily controlled. In a process for manufacturing an ink jet head of a so-called side shooter type especially having a discharge port above an electrothermal conversion member, which is a pressure generating element, among ink jet heads, since the discharge port is formed in the coating resin layer for forming the ink flow path wall, the film thickness of the ink flow path wall becomes a factor to determine the distance between the electrothermal conversion member and the discharge port, which affects discharge characteristics. Therefore, the formation of the coating resin layer for forming the ink flow path wall in the process for manufacturing an ink jet head of the side shooter type is often performed by spin coating. As described above, when the coating resin layer for forming the ink flow path wall is formed by solvent coating, since a positive type resist is provided in the above-described ink flow path pattern as a soluble resin layer, it is required to use a carefully selected solvent of the coating resin for forming the ink flow path wall. Specifically, if the dissolving power of the solvent used in solvent coating is excessively strong, an unexposed portion of the positive type resist soluble in this solvent may be partially dissolved, and in this case, there is a problem in that the deformation of the obtained ink flow path wall is caused.
In order to make the thickness of the film formed on the substrate for the ink jet head by solvent coating represented by the above-described spin coating uniform, it is required to adjust the evaporation rate of the solvent, the adjustment of the viscosity of the solvent, and the like are required. In particular, the film in the field of ink jet heads is formed considerably thicker than the film thickness in the field of normal semiconductors, and it is difficult to make the film thickness uniform unless various film-forming conditions are more closely controlled than the films in the field of semiconductors. Also since the film thickness of the above-described ink flow path pattern affects the discharge characteristics, the control of the evaporation rate and the viscosity significantly affects the yield of ink jet heads. In particular, regarding to the evaporation rate of the solvent, the solvent having a lower evaporation rate can more easily achieve the equalization of the film thickness. However, since such solvents with a low evaporation rate have often high solubility, in the conventional processes of manufacturing ink jet heads, if a solvent having high solubility is used as the solvent for applying the resin when the ink flow path wall is formed, the deformation of the ink flow path wall may occur, the problem of yield may arise, and method does not necessarily lead to the improvement of productivity.
As a method to solve such problems, in Japanese Patent Application Laid-Open No. H8-323985, a process wherein a photosensitive resin layer of an ionizing radiation decomposition type containing crosslinkable structural units is formed in the portion to become an ink flow path; a coating resin layer for forming an ink flow path wall coating the resin layer; and the above-described soluble resin layer is removed; is disclosed. According to this process, since the positive type resist forming the ink flow path is not dissolved by the coating resin forming the ink flow path wall, there is no possibility of the deformation of the ink flow path wall. However, with the advance of ink jet printers in recent years, an ink jet printer is required to stably discharge fine liquid droplets that enable high picture-quality recording, and therefore, a more minute ink jet head must be manufactured. Here, if an ink jet head having a more complicated structure is manufactured using the process described in Japanese Patent Laid-Open No. H8-323985, in the patterning of the ink flow path forming member, there are problems of:    (1) since the distance between crosslinked molecules is short, and the rigidity of the resin rises, cracks may be produced due to solvent shock when an extremely complicated pattern portion is formed;    (2) since the degree of freedom of crosslinking points is limited, the crosslinked density does not elevate, and since the resin is swollen by the developer during development, cracks may be produced due to sharp difference of stress; and    (3) some ink flow path forming members must be baked in the intermolecular crosslinking step at a temperature as high as 180° C. to 200° C. for a long time, and has a problem in the improvement of productivity; and there may also be problems in the narrowed design width of the ink flow path, and in the improvement of productivity.
As described above, in the prior art, there have been problems of difficulty to further improve yield and productivity while forming a highly accurate ink flow path, in the process for manufacturing an ink jet head comprising steps of providing a photosensitive resin layer contributing to the formation of the ink flow path on a substrate for the ink jet head, providing a coating resin layer for forming an ink flow path wall on the photosensitive resin layer, and then, forming the ink flow path by dissolving and removing the photosensitive resin layer in the portion of the ink flow path.